The basic image-forming process of silver halide color photography comprises the exposure of a silver halide photographic recording material to actinic radiation (such as light) and the manifestation of a useful image by wet chemical processing of the material. The fundamental steps of this wet processing include color development to reduce silver halide to silver and to produce dye images in exposed areas of the material. During or after bleaching to oxidize metallic silver to silver(I), the silver(I) is generally removed by dissolving it in a silver(I) solvent, commonly known as a fixing agent. Conventional fixing steps generally require up to 6 minutes in large photoprocessing operations, and up to 2 minutes in small "minilabs" or small processing machines.
In some photochemical processes, bleaching and fixing are combined in a bleach-fixing step using a composition that includes both a bleaching agent to oxidize metallic silver and a fixing agent to dissolve the remaining silver(I).
A wide variety of fixing agents and silver solvents are known, as described for example in U.S. Pat. No. 5,633,124 (Schmittou et al) and publications noted therein. Thiosulfate salts are generally preferred as fixing agents because they are inexpensive, highly water soluble, non-toxic, non-odorous, and stable over a wide pH range. Thus, fixing is usually accomplished using a thiosulfate fixing agent that diffuses into the element, and forms silver thiosulfate complex that diffuses out of the element. In large photofinishing labs, the elements are usually immersed in a fixing solution for from 4 to 6 minutes. In small minilabs, fixing time is shorter, that is from 90 to 120 seconds.
In processing some photographic elements, such as color negative photographic films, there is a need to reduce density from dye stain resulting from dye aggregates formed from sensitizing dyes commonly included in the elements to increase silver halide spectral sensitivity. After the photographic elements are exposed, the spectral dyes are no longer needed, and the aggregates they form interfere with the absorption characteristics of the colored dyes that provide the final color images.
When using conventional fixing times, the unwanted dye aggregates disappear after the prescribed lengthy fixing and stabilizing (or washing) steps. However, when the fixing time is shortened, dye aggregates and resulting dye stains remain. This problem in the original image (such as color negative film images) is unacceptable in the photographic industry. It is also unacceptable for such images as color slides or transparencies, color prints or electronic images obtained from scanning original images.
It is well known that the rate of silver dissolution (or complexation) by thiosulfate fixing agents increases with increasing thiosulfate concentration until a maximum rate is reached. After this maximum rate is reached, the rate of silver dissolution decreases as the thiosulfate concentration is increased further. Consequently, other compounds are routinely incorporated into fixing solutions to act as co-fixing agents or fixing accelerators to improve silver removal. Thiocyanate is one of the most common compounds used for this purpose.
There are several fixing solutions available in the marketplace containing a combination of thiosulfate and thiocyanate for use in a 90-120 second fixing step. One such product is available as KODAK FLEXICOLOR RA Fixer Replenisher NR having thiosulfate and thiocyanate at 0.8 and 1.2 mol/l, respectively (1:1.5 molar ratio).
There are also numerous literature references to the combination of thiosulfate and thiocyanate including EP-A-0 610 763 (Buttner et al) that describes fixing for 90 to 240 seconds. However, this publication fails to appreciate the need to avoid sensitizing dye aggregate stains.
There is a continuing need to provide images in photographic elements in a more rapid fashion. The industry is attempting to provide images to customers in less time, and thus a time reduction in any of the processing steps, including fixing, is highly desirable. The combination of thiosulfate and thiocyanate in conventional amounts is insufficient to complete the desilvering process in a more rapid fashion while eliminating unwanted dye stain from sensitizing dye aggregates.